Problem: On the first 3 tests of his geometry class, Michael got an average score of 81. What does he need on the next test to have an overall average of 84?
Explanation: Let his score on the next test be $x$ The sum of all of his scores is then $3 \cdot 81 + x$ The same sum must also be equal to $4 \cdot 84$ Solve: $x = 4 \cdot 84 - 3 \cdot 81 = 93$.